Becker muscular dystrophy

Becker muscular dystrophy (BMD) is caused by specific mutations in the DMD gene. The DMD gene gives the body instructions to make a protein called dystrophin. This protein helps stabilize and protect muscle fibers and may play a role in chemical signaling within cells. Mutations that lead to an abnormal "version" of dystrophin that allow it to keep some of its function usually cause BMD. Muscle cells without fully functional dystrophin become damaged as muscles contract and relax with use. They then weaken and die over time, leading to the muscle weakness and heart problems in people with BMD.

Becker muscular dystrophy (BMD) may first be suspected in a person with signs or symptoms of BMD. Healthcare providers will often conduct neurological and muscle exams, as well as order specific laboratory tests. A careful medical history is also important to differentiate between BMD and Duchenne muscular dystrophy.

Exams in a person with BMD may reveal:

• Abnormally developed bones, leading to deformities of the chest and back (scoliosis)
• Abnormality of heart muscle function (cardiomyopathy)
• Congestive heart failure or irregular heartbeat (arrhythmias)
• Muscle deformities
  • Contractures of heels and legs
  • Fat and connective tissue (pseudohypertrophy) in calf muscles
• Muscle wasting that begins in the legs and pelvis, then progresses to the muscles of the shoulders, neck, arms, and respiratory system

Laboratory tests that help confirm the diagnosis include:

• CPK
• Electromyography (EMG)
• Muscle biopsy or genetic testing

Becker muscular dystrophy is inherited in an X-linked recessive manner. A condition is considered X-linked if the mutated gene that causes the condition is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), one mutated copy of the gene in each cell is enough to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the condition. Males are affected by X-linked recessive disorders much more frequently than females. A specific characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

In X-linked recessive inheritance, a female with one mutated copy of the gene in each cell is called a carrier. Female carriers of X-linked recessive conditions have a 50% (1 in 2) risk to pass on the mutated gene to each child. Male children have a 50% risk to be affected, and female children have a 50% risk to be a carrier. Female carriers usually do not have signs or symptoms of the condition. Occasionally, females who carry a DMD mutation may have muscle weakness and cramping. These symptoms are typically milder than the severe muscle weakness and wasting in affected males. Females who carry a DMD mutation also have an increased risk to develop heart problems, including dilated cardiomyopathy.

In about two thirds of cases, an affected male inherits the mutation from his mother who carries a mutated copy of the DMD gene. The other third of cases probably result from new mutations in the gene.

Becker muscular dystrophy (BMD) generally leads to slowly worsening disability, but the amount of disability can vary among affected people. Some men need a wheelchair, while others may only need walking aids such as canes or braces. The lifespan is often shorted due to heart disease and respiratory complications. Most people with BMD survive well into mid- to late adulthood. If the effects of the condition on the heart are mild, or if they are adequately controlled with medical intervention, a person can have a normal or nearly normal life span.

The incidence (rate of occurrence of new cases) of Becker muscular dystrophy (BMD) has been estimated to be between 1 in 18,000 and 1 in 30,000 male births.

Prevalence estimates (all the people affected with BMD at a given time) are relatively broad and the ranges differ depending on the source. According to Medscape Reference, the prevalence of BMD is estimated to be between 17 to 27 cases per million people. This means that at a given time, about 1 in 37,000 to 1 in 59,000 people are living with BMD. According to Orphanet, the prevalence is estimated to be between around 1 in 11,000 to 1 in 100,000.

There is currently no cure for Becker muscular dystrophy (BMD), and management aims to help with symptoms and improve the quality of life. Affected people are encouraged to remain active, because inactivity (such as bed rest) can make the muscle disease worse.

Physical therapy can help with stretching tight muscles and using assistive devices; occupational therapy can help with daily living skills; and speech therapy may help those with dysphagia (difficulty swallowing). Surgery may be needed for progressive scoliosis and development of contractures.

People with BMD should be monitored for orthopedic complications. Cardiac (heart) evaluations are recommended beginning at around 10 years old, or when symptoms first begin. Evaluations should be repeated at least every two years.

Some studies have shown that certain corticosteroids (such as prednisone or prednisolone) can slow the decline of muscle strength in people with Duchenne muscular dystophy; however, information about their use in people with BMD is limited. There are a number of additional therapies for BMD being studied. Potential future treatments for BMD may include gene therapy, exon skipping, ataluren, creatine, deacetylase inhibitors, myostatin inactivation, and cell therapy (myoblast treatment, and/or the use of stem cells).